As a conventional exhaust emission control system for efficiently removing particulate matters (hereinafter referred to as PM) exhausted from an internal combustion engine, a system using a diesel particulate filter (hereinafter referred to as DPF) has heretofore been proposed. Unless the captured PM is removed in the system using the DPF, the filter is finally clogged, and therefore the filter needs to be periodically regenerated.
In general, the regeneration of the filter is possible, when the DPF is heated and burnt. However, for example, an exhaust gas temperature of a diesel engine does not easily reach to a burning temperature of the PM. Therefore, there has been considered a method in which the temperature of the DPF is raised by external heat sources such as an electric heater and a burner to burn soot that is a main component of the PM, or a method in which the DPF is periodically replaced, and the removed DPF is heated by an electric furnace.
However, in the method in which the filter is heated by the external heat sources such as the electric heater and the burner, the PM deposited on the filter can be comparatively stably burnt, but the electric heater or the combustion burner is a complicated/expensive device, and therefore this method has been restricted to some special applications. In the method in which the DPF is periodically replaced, handling of the filter has been troublesome.
To solve the problem, there has been proposed a system in which a time to supply fuel to an internal combustion engine, for example, a fuel injection time in a diesel engine or the like is adjusted to raise an exhaust gas temperature, and the PM deposited on the filter is periodically burnt. This system is sometimes combined with a method for lowering the burning temperature of the PM in order to more sufficiently burn the PM. Since the method is a comparatively simple method, it has also been possible to mount an exhaust emission control system in which the DPF is used in removing the PM exhausted from the diesel engine mounted on the automobile.
However, in the method in which the PM is deposited on the filter, and the injection time of the diesel engine is adjusted at a certain time interval to raise the exhaust gas temperature, the exhaust gas temperature rises in a remarkably short time. Therefore, as compared with the case where the PM is burnt by the electric furnace or the like, a rapid temperature rise in the filter at the time of regeneration easily occurs, additionally a temperature gradient in the filter increases, and there has been a possibility that the filter is disadvantageously cracked. Especially in the method for lowering the burning temperature of the PM, activation energy of the PM is lowered. Therefore, as compared with a case where the PM is simply burnt by the exhaust gas temperature rise from the engine, the PM is burnt in a comparatively short time, thermal energy received by the DPF per unit time increases, an excessive temperature distribution is made in the DPF, and there has been a possibility that the DPF is more easily cracked or that materials are molten.
On the other hand, the filter has a mechanism in which openings of cells of a honeycomb structure constituted of a porous ceramic are alternately sealed, and the PM is captured during passing of the exhaust gas through porous honeycomb walls. During the passing of the exhaust gas through the walls, emission resistance is generated, and there is a problem that a difference in pressure loss before/after the filter increases. When the pressure difference increases, an output from the engine drops. Therefore, to obtain the same performance, as compared with a case where the filter is not mounted, much more fuel is required. When the pressure difference excessively increases, there is a possibility that the fuel does not burn well in the engine and the engine does not operate disadvantageously. Therefore, it is an important function of the filter to reduce the pressure loss difference before/after the filter.
Moreover, when a fuel auxiliary agent is used for lowering the burning temperature of the PM, and when the fuel auxiliary agent burns together with the PM, a large amount of ashes are generated, and there is a problem that a back pressure before/after the filter increases. Even in a method in which a catalyst is imparted to the filter, the catalyst itself or a washed coating of the catalyst closes pores in the filter, and therefore there is a problem that the pressure loss in the filter increases. Therefore, in the exhaust emission control system, it has been necessary to suppress the increase of the pressure loss to the utmost while well suppressing a filter temperature at the time of regeneration.
On the other hand, various methods of calculation of the pressure loss of the filter have heretofore been proposed, but the pressure loss can be calculated well on conditions that any PM is not deposited on the filter, but it has not been possible to calculate the pressure loss in a case where the PM is deposited. Therefore, since it is difficult to calculate the pressure loss of the filter at the time of actual use in the exhaust emission control system with good accuracy, it is difficult to predict an optimum filter structure in which the pressure loss is well balanced with the filter temperature at the time of regeneration. There has been a demand for a method of calculating the pressure loss with good accuracy in order to obtain the exhaust emission control system provided with the above-described optimum filter.